1. Field of Invention
The present invention relates to a planarization process in semiconductor manufacture. More particularly, the present invention relates to a post chemical-mechanical polishing (CMP) cleaning of a silicon wafer.
2. Description of Related Art
In the manufacturing of deep submicron semiconductors, a copper damascene process to form a conductive line in a low dielectric constant (low-k) inter-metal dielectric layer is an effective method of lowering resistor-capacitor (RC) delay and resisting electromigration resistance. In a damascene process, one important step is copper chemical-mechanical polishing (Cu-CMP) for the removal of excess copper above a dielectric surface.
Chemical-mechanical polishing is an operation that utilizes the mechanical grinding action of a polishing wheel and the chemical action of a suitable chemical agent to planarize the undulating surface profile of a silicon wafer. Principle components of a chemical-mechanical polisher include a polishing table for grinding the silicon wafer and a handle for grasping the back side of the silicon wafer. To carry out a chemical-mechanical polishing, the back side of the wafer is grasped by the handle while the front side of the wafer is pushed against a polishing pad on the rotating polishing table. Chemical agents necessary to assist the polishing such as slurry is delivered along a pipeline system to the polishing pad. Utilizing the abrading action of the polishing pad and the chemical action of the slurry, the front surface of the wafer is planarized.
However, during a chemical-mechanical polishing operation, contaminants that originate from the slurry or abrasive particles in the polishing pad may settle on the wafer surface after polishing. In the polishing of a copper/low dielectric constant material on a silicon wafer, by-products or carbon-rich particles may also settle onto the wafer surface after polishing.
In a copper chemical-mechanical polishing operation, a barrier layer or an etching stop layer on the low dielectric constant material layer are often used as a stop layer in a barrier chemical-mechanical polishing (barrier-CMP) operation. However, the process frequently leads to a dishing of the surface of copper conductive lines and an exposure of the low dielectric constant material near the edges of the copper lines. Consequently, a portion of the low dielectric constant material is likely to be polished away, generating large quantities of carbon-rich particles on the wafer surface. In addition, because the carbon atoms within most types of neutral-to-acidic slurry have a potential opposite to that of a copper surface, the carbon-rich particles are likely to attach to the copper surface, producing surface defects.
To remove the contaminants on a silicon wafer, a cleaning step is often added after chemical-mechanical polishing. At present, most integrated circuit manufacturers opt for cleaning the wafer with an aqueous chemical solution or de-ionized (DI) water in combination with some form of brushing, jetting or ultrasound. De-ionized water removes the contaminants by the application of external forces. Aqueous chemical solution removes the contaminants by attacking the wafer surface or reacting with the contaminants before removing the dislodged contaminants from the wafer.
However, cleaning a wafer with de-ionized water cannot remove all surface contaminants. On the other hand, using an aqueous chemical solution to remove contaminants may damage the wafer surface. Moreover, some of the contaminants may be chemically inert to the chemical ingredients in the aqueous solution. For example, carbon-rich particles or chemical reaction by-products attached to the wafer may not be easily removed by the chemicals in the aqueous solution.
Methods related to the removal of contaminants on a silicon wafer after chemical-mechanical polishing can be found in various articles. For example, N. Miyashita et al. have written an article titled, “A new post-CMP cleaning method for trench isolation process in CMP” (MIC conference, 1996). The proposed method involves using an activated interfacial reagent to carry out chemical-mechanical polishing so that the surface of the polysilicon film remains hydrophilic after chemical-mechanical polishing and waste particles are removed in-situ.
All the aforementioned conventional cleaning methods are inefficient in removing most contaminants from a silicon wafer and maintaining post CMP wafer surface properties. Hence, semiconductor manufacturers are still looking for an effective and economic post-CMP cleaning that can effectively remove contaminants from a wafer surface with as few changes in surface properties as possible.